This invention relates to the removal of carbonyl sulfide from gaseous process streams which contain such contaminant in a concentration greater than 1 part per million, by volume. More particularly, the invention relates to the treatment of such process streams with adsorbent beds containing zinc oxide to effect a near complete removal of the carbonyl sulfide in such streams.
Synthesis gas is an increasingly important feedstock in the chemical industry. Existing or proposed commercial processes using synthesis gas (i.e. gaseous mixtures containing hydrogen and carbon monoxide) include processes for the manufacture of methanol, ethanol, the production of aldehydes by the oxo process, the production of glycols using rhodium catalysts, and the production of purified hydrogen and carbon monoxide streams. In most of these processes, the use of sensitive catalyst materials requires that contaminants such as sulfur compounds and hydrogen cyanide be removed from the gas to concentration levels of less than 1 part per million, by volume (hereinafter referred to as "ppmv"), and often to levels below 0.1 ppmv.
Synthesis gas mixtures typically contain a variety of impurities among which are sulfur compounds such as hydrogen sulfide (H.sub.2 S), carbonyl sulfide (COS), sulfur dioxide (SO.sub.2), carbon disulfide (CS.sub.2) and methyl mercaptan (CH.sub.3 SH), as well as hydrogen cyanide (HCN), hydrogen chloride (HCl) and others. The relative concentrations of these impurities in the gas depends on the feedstock from which the synthesis gas is derived. Generally, a gaseous feedstock, such as methane, introduces less contaminants into the synthesis gas than liquid feedstocks, such as naptha, gas oil, atmospheric residue (the bottom fraction obtained from an atmospheric crude refining still) and vacuum residue (the bottom fraction obtained from the vacuum refining of heavy feedstocks such as crude oil and atmospheric residue). Coal derived synthesis gas generally contains the highest concentration of sulfur compounds.
Present purification schemes typically utilize a reactive liquid absorbent such as aqueous ethanolamines, alkali carbonates and hydroxides, or sodium thioarsenite as a primary purification agent to absorb high levels of the various species of impurities and reduce them to levels of about 1 to 10 ppmv. Alternatively, a non-reactive physical absorbent such as methanol at cryogenic temperatures may be used as the primary purification agent. Purifying the gaseous stream to a higher degree with such absorbents is uneconomical because of the disproportionately large amounts of energy which would be required to regenerate the spent absorbent.
Accordingly, the effluent gas from a primary purification step usually requires further treatment to reduce the impurities to acceptable levels. Adsorbents to accomplish such purification are extensively described in the prior art. The prior art literature relating to adsorbents for gaseous purification concerns itself, for the most part, with eliminating sulfur compounds from gas streams, in particular H.sub.2 S. Thus, for example, U.S. Pat. No. 3,441,370 describes the removal of H.sub.2 S with the use of a zinc oxide adsorbent at a temperature from ambient to 800.degree. F. The removal of COS and RSH is also suggested, but only at temperatures above 500.degree. F. However, no data is provided in the patent to demonstrate the removal of COS with such adsorbent. U.S. Pat. No. 4,009,009 describes the removal of COS from arsenic-free gas streams with the use of alumina-supported lead oxide. Great Britain Application No. 012,540, filed Mar. 29, 1976 (corresponding to German Offenlegungsschrift No. 2,650,711) discloses the use of zinc oxide as an absorbent for hydrogen sulfide. The examples of the Application show the removal of carbonyl sulfide along with H.sub.2 S, but the presence of carbonyl sulfide in the inlet feed gas is said to be restricted to small amounts (page 4, col. 2). Thus, in the examples of the British Application, the maximum amount of COS in the gaseous feed stream to the adsorbent was 0.4 ppmv, as compared to the maximum amount of H.sub.2 S which was 200 ppmv. Moreover, the examples of the application are deserving of further comment with regard to the ambiguity of the data concerning the removal of COS with zinc oxide. In Example I, the COS concentration in the feed gas to the zinc oxide adsorber was 0.13 ppmv and the exit concentration was 0.12 ppmv, a value essentially the same as the inlet concentration and within the experimental error of the measurement. In another experiment in Example I, the inlet concentration of COS was reported as 0.09 ppmv and the exit concentration was 0.11 ppmv, an apparent increase in the COS concentration. In Example II, two feed gases were used having COS concentrations of 0.07 and 0.04 ppmv, respectively, and the effluent gases were reported to have COS concentrations below these values. In Example III, the concentration of COS was reported as 0.4 ppmv in the feed gas and as 0.03 ppmv in the product gas. The erratic nature of the above-described data indicates that rather than being removed by adsorption on zinc oxide, COS was removed by adsorption on the metal surfaces of the tubing and the reactor which contacted the feed gas. Since it is known that COS is readily adsorbed on metal surfaces in amounts comparable to the very low COS concentrations which were present in the inlet feed gas, the data of these examples indicate nothing more than the expected removal of COS by adsorption on metal. Thus, the British Application does not disclose the efficacy of zinc oxide as an adsorbent for COS.
U.S. Pat. No. 3,492,083 broadly describes the removal of H.sub.2 S and COS from an industrial gas using as an adsorbent a mixture comprising oxides of aluminum, zinc, iron and/or manganese in combination with oxides of the alkaline earth and/or alkali metals. Adsorption is carried out at a temperature of from 100.degree. to 300.degree. C. The examples of the patent only disclose the removal of H.sub.2 S and SO.sub.2 from the various gases. U.S. Pat. No. 4,128,619 discloses a desulfurization process carried out at a temperature from 100.degree.-400.degree. C. using zinc oxide as the adsorbent. Hydrogen sulfide is the only sulfur compound which is shown removed in the examples of the patent. U.S. Pat. No. 2,239,000 discloses the removal of sulfur from the gas mixtures comprising hydrogen and carbon monoxide at a temperature from 400.degree. C.-600.degree. C. using catalytic mixtures of zinc and iron oxides or zinc and chromium oxides.
Thus, while zinc oxide is generally known in the prior art as an adsorbent for the removal of H.sub.2 S from gaseous streams, there has heretofore been no suggestion regarding its capability as an adsorbent for COS at temperatures below 500.degree. F.